(a) Technical Field
The present invention relates to a low temperature co-fired ceramic (LTCC) composition with low dielectric loss for millimeter-wave application.
(b) Background Art
Wireless communication technology has been recently expanding its territory from simple voice communication services to complex and various multimedia services including video broadcasting, video telephone and file transfer. Although the millimeter wave bands of the extremely high frequency region have been reserved for special wireless communications including military use, the competition and saturation in the GHz frequency region requires their extension to millimeter wave bands such as 60 GHz. The use of the frequency region is expected to increase further in the future.
At present, leading companies in this field including DuPont, Ferro, Heraeus, and Kyocera have been developing dielectric materials and components for electronic devices using the millimeter-wave frequencies. DuPont and Ferro have already acquired essential techniques on various low temperature co-fired ceramic (LTCC) compositions. Particularly, DuPont's DP 943 dielectric tape and Ferro's A6 dielectric tape are thought as key materials for millimeter-wave application by many passive component producers. IMST of Germany and VTT of Finland also have technologies vital to components for millimeter-wave applications, including high-level integration modules for LMDS in the 24-GHz band, radar sensors for vehicles in the 77-GHz band, or the like. Heraeus is currently developing dielectric materials for the extremely high frequency band based on its own technology in the field of new materials. In Japan, NEC has developed a wireless-HD transceiver using a dielectric material for millimeter-wave application with extremely low loss. Hitachi announced that it has completed the development of a radar transceiver for vehicles in the 77 GHz band. Kyocera has been selected as Intel's WiMAX RF FEM provider with its high temperature coefficient of expansion (TCE) LTCC technology.
The LTCC technology is a key technology in developing dielectric materials for millimeter-wave application. The core of the LTCC technology for production of miniaturized, high-functionality electronic devices is to realize an integrated, modularized circuit board. The LTCC dielectric material is sintered at low temperature of 950° C. or less, thus enabling to use silver (Ag) as an internal electrode. This can reduce the cost as compared to the internal electrode made of expensive precious metal such as platinum (Pt) while providing better electric conductivity. To allow the sintering at low temperature, the sintering temperature of the ceramic dielectric material needs to be lowered. In general, glass frit is used for this purpose. Low dielectric loss is particularly required for a dielectric composition for millimeter-wave application. However, since the addition of glass frit results in a low quality factor in most cases, the addition amount of the glass frit is restricted.
The commercially available glass frit compositions used as LTCC dielectric compositions include: the anorthite (CaMgSi2O6)-based glass composition developed by Motorola, the calcium borosilite-based glass composition of Ferro, and the lead borosilicate-based glass composition of DuPont are typical examples. These glass frit materials are not marketed alone but incorporated into LTCC dielectric compositions. In order to reduce dielectric loss of these dielectric compositions, the glass is crystallized or it is mixed with a dielectric ceramic filler with high quality factor. Examples of the dielectric ceramic filler with high quality factor include oxide compounds such as MgAl2O4, ZnAl2O4, etc.
Korean Patent No. 0704318 describes an LTCC dielectric composition with low dielectric loss, including 15-55 wt % of one or more oxide filler selected from ZnAl2O4, MgAl2O4, ZrSiO4 and Mg2SiO4, having a high quality factor at 1 MHz. It is described that samples prepared from the composition have a dielectric constant 4-7, a dielectric loss of 0.02-0.2% and a quality factor of 500-5000, at 1 MHz.
Korean Patent Application Publication No. 2003-0042957 describes that CaWO4—Mg2SiO4 can be used as a dielectric ceramic filler in the millimeter wave band. Similar dielectric materials with a low dielectric constant are described for example in U.S. Pat. Nos. 6,835,682; 5,258,335; 5,902,758; and 7,160,823.